Significant events
- Y-chromosomal Adam: 200,000-300,000 ya
- Mitochondrial Eve: 120,000-156,000
- Invention of fire: 1,700,000-200,000
- Behavioral modernity (abstract planning, symbolic behavior, large game hunting): 40,000-50,000
- Literacy (See: Evolution of Literacy): 10,000
- Beringia submerged: 10,000
Genetic escape velocity
There is a genetic escape velocity, whereby you produce enough descendants such that it would be nearly impossible for you to have no descendants at any arbitrary time in the foreseeable future. Abraham Lincoln, for example, did not achieve genetic escape velocity, since his bloodline ended after 10 descendants spread over 3 generations.
Abraham Lincoln had four children, only one of which survived into adulthood. One son, Robert Todd became a railroad tycoon and had three children. Of those three children, two had children, leaving behind three great-grandchildren who did not have children. All told, Lincoln had 10 descendants over 3 generations, in an ultimately terminating bloodline.
Genealogical math
The MRCA didn't necessarily pass any genetic information to everyone. After 32 generations, a contribution of a single ancestor is 2^-32, or 1/(4 billion). There are only ~3 billion basepairs in the human genome.
Assuming no pedigree collapse, a single individual alive today, would over 30 generations going back to High Middle ages have 2^30, or ~1 billion ancestors, more than the total world population at the time.
Important ancestors
MRCA
The picture of the Most Recent Common Ancestor (MRCA) is that of some sexually productive person whose children, grandchildren, and great-grandchildren dispersed far and wide, eventually trickling through all family trees in Eurasia, Africa, the Americas, and Australia. Likely they lived near Alaska, Siberia, or Eastern Asia, in order to have produce descendants who traveled to the Americas and Australia before their bridges closed.
The MRCA of humans is the most recent individual (or possibly a couple, if they were exclusive mates) from whom all humans are descended from. MRCA existed in some tribe, and their contemporaries are ancestors to some humans today, but none of those contemporaries are connected to all of us. If they did, and they mated more recently, then they would be the MRCA.
Certainly they were alive before Beringia closed (10 ka) and the Australian landbridge closed (12 ka), whichever came first, leaving descendants in the Americas, Eurasia, and Australia.
The Founding Cohort
In the popular imagination, there exists a population bottleneck that is the ancestor of all humans. This population is imagined as an extended tribe or group of 50,000 or so humans whose descendants stood out among the hominids and eventually dominated the rest. The ACA, or All Common Ancestors point, appears to represent this.
The ACA is the point at which everyone was an ancestor to everyone alive today. There were no partial ancestors. The ACA may not even exist, though, and it appears to have shaky standing.
"Adam" or "Eve"
Another popular ancestor is that of "Eve." "Mitochondrial Eve" is technically just the matrilineal MRCA, not an Eve the way the Bible would describe that. A true Eve would have to be further back to some hominid whose contemporaries all radiated out to other species, living or extinct. Mitochondrial Eve, going by the matrilineal MRCA definition, had too many contemporaries who were ancestors of many of us today, although not all of us.
